Which box-and-whisker plot represents the data set?11, 6, 19, 14, 21, 7, 13, 15, 15

The box and whisker plot is attached.

We first order the data from least to greatest:
6, 7, 11, 13, 14, 15, 15, 19, 21

The median is the middle value, or 14.
The lower quartile is the median of the lower half (split by the median).  This is between 7 and 11: (7+11)/2 = 18/2 = 9
The upper quartile is the median of the upper half (split by the median).  This is between 15 and 19:  (15+19)/2 = 34/2 = 17
The highest value is 21.
The lowest value is 6.

We draw the middle line of the box at 14, the median.  We draw the left side of the box at the lower quartile, 9.  We draw the right side of the box at the upper quartile, 17.  From the right side of the box, we draw a whisker to the highest value, 21. From the left side of the box, we draw a whisker to the lowest value, 6.